Fill the moka pot base with water to just below the safety valve. Fill the basket level with coffee without tamping, assemble the pot, and heat over medium-low.
Make the Espuma:
Place sugar in a small bowl. As soon as the first teaspoon or two of dark coffee emerges, spoon it over the sugar.
Beat vigorously for 2 to 3 minutes until the sugar becomes pale, thick, and glossy.
Finish:
Pour the remaining brewed coffee into a heatproof cup. Fold in the whipped sugar until a thin caramel-colored foam rises.
Divide immediately among six demitasse cups.
Notes
Do not tamp coffee in a moka pot; level it gently for safe, even brewing.
Traditional cafe cubano is intentionally sweet and served in very small portions.